1. Field of the Invention
This invention relates to an ignition apparatus for an internal combustion engine, and more particularly to a laser ignition apparatus for an internal combustion engine which causes ignition of the air-fuel mixture by a laser beam with a high energy density.
2. Description of the Prior Art
In a usual ignition apparatus for an internal combustion engine, a high voltage is applied to an ignition plug which is fixed on the wall surface of a combustion chamber in order to ignite the air-fuel mixture by spark discharge. In an ignition apparatus of this kind, since the ignition plug is exposed directly to the combustion chamber, carbon generated attaches to an insulator of the ignition plug to render the discharge of the ignition plug difficult. Furthermore, due to a heat loss of the electrodes of the ignition plug, a torch or nucleus of flame generated by the discharge is cooled, and vanished before reaching a flame. Since the ignition occurs on the wall surface of the combustion chamger, the condition of the air-fuel mixture is difficult to be ignited than at the center part of the chamger. Even if it is ignited, it takes a considerable time before the flame spreads over the whole space of the combustion chamber. In order to circumvent these defects, an ignition plug has been proposed, in which the electrodes are made to protrude into the center part of the combustion chamber. However, there have been still problems of durability of electrodes and of preignition in which the combustion precedes the ignition timing.
In order to overcome these problems, a laser ignition apparatus using a light beam with a high energy density such as laser has been proposed, in which the light beam is focussed on a predetermined position in the combustion chamber for ignition. In this method, the laser beam is directly irradiated on the air-fuel mixture and raises the temperature of the gas molecules thereby to cause ignition. However, since the rate of light absorpotion of the air-fuel mixture is small, a practical difficulty arises in ignition. Therefore, another method has been proposed, in which a laser beam causes, at first, breakdown of gas and then the produced plasma ignites the ambient air-fuel mixture. In order that the gas breakdown by the laser beam is ensured, the energy density should be above 10.sup.9 to 10.sup.10 W/cm.sup.2 at a normal pressure under which the ignition occures. Although the energy density can be increased by decreasing the size of the focal point of the laser beam, there is a limit on the efficiency. For example, if the diameter of focal point is 50 .mu.m, an output of the order of 8.times.10.sup.4 to 8.times.10.sup.5 W is necessary from an atmospheric pressure to 10 atm. Such a large output can not be realized by a CW laser. A pulsed laser is therefore used and the output is enhanced by Q switching. In this case, the pulse width of the laser is usually very short, say less than 10.sup.-7 sec (100 nsec). The high temperature plasma produced by the Q switching pulsed laser diffuses rapidly with time. From a viewpoint of ignition phenomenon, a period of 100 .mu.sec to 1 msec is usually needed before the flame nucleus is formed and grows flames. It is necessary that the energy is supplied intermittently or continuously to the flame nucleus. In order to ignite the air-fuel mixture by a single short pulse of the laser, the energy should be larger than the value required for the gas breakdown. This is not preferrable in view of the energy efficiency.